This application relates generally to FT-IR (Fourier transform infrared spectroscopy) and more specifically to phase-modulated (PM) photoacoustic spectroscopy (PAS) measurements.
A Fourier transform spectrometer typically includes an interferometer into which are directed an infrared beam to be analyzed and a monochromatic (laser) beam that provides a position reference. The interferometer has first and second mirrors.
Each of the input beams is split at a beamsplitter with one portion traveling a path that causes it to reflect from the first mirror and another portion traveling a path that causes it to reflect from the second mirror. The portions of each beam recombine at the beamsplitter, and the recombined beams are directed to appropriate detectors. The difference between the optical paths traveled by the first and second portions of the beams is often referred to as the retardation or retardation value.
One of the mirrors (referred to as the fixed mirror) is fixed or movable over a limited range while the other mirror (referred to as the movable mirror) is movable over a much more extensive range. In rapid scanning, the retardation is changed at a nominally constant rate over a significant range. This is typically accomplished by moving the second mirror at a nominally constant velocity. In step scanning, the retardation is changed intermittently, in relatively small steps of retardation. In some implementations, this is accomplished by stepping the movable mirror position.
The optical interference between the two beam portions causes the intensity of the monochromatic beam and each frequency component of the infrared beam to vary as a function of the component's optical frequency and the retardation. The detector output represents the superposition of these components and, when sampled at regular distance intervals, provides an interferogram whose Fourier transform yields the desired spectrum.
The monochromatic beam provides a reference signal whose zero crossings occur each time the relative position between the fixed and movable mirrors changes by an additional one quarter of the reference wavelength (i.e., for each half wavelength change of retardation). The data acquisition electronics are triggered on some or all of these zero crossings to provide regularly sampled values for the interferogram.
In a step scan interferometer, the relative position between the fixed and movable mirrors is stepped from one retardation value to the next and then held, at which point an intensity measurement is made. The sequence is then repeated until the desired interferogram has been acquired. The prior art teaches various techniques for accomplishing this under servo control. A number of approaches are disclosed in U.S. Pat. No. 5,166,749, issued Nov. 24, 1992 to Raul Curbelo et al., for STEP SCANNING TECHNIQUE FOR INTERFEROMETER, which is incorporated by reference in its entirety for all purposes. This patent discloses an implementation of step scanning where the movable mirror is driven at a constant velocity and the "fixed" mirror is driven, using an actuator such as a piezoelectric transducer (PZT), in a sawtooth fashion over a small distance corresponding to the desired step size. The superposition of the two movements results in a stepped retardation.
In photoacoustic spectroscopy (PAS), a sample is placed in a cell with an infrared-transmissive window on one side and a microphone on the other. The sample is surrounded by a gas that does not absorb infrared radiation. A pulse of infrared radiation is directed at the sample, which absorbs the infrared radiation in accordance with the sample's infrared spectral characteristics. The absorbed infrared energy heats the sample, and the heat is transferred from the sample to the gas. This causes pressure changes in the gas, which are detected by the microphone. In a step scanning FT spectrometer, the process is repeated as the retardation in the spectrometer is stepped along a series of values.
The time dependence of the pressure changes provides information on the internal structure of the sample (e.g., a particular spectral feature at a particular depth in the sample). The output signal for each step is, in effect, a convolution of the excitation pulse shape, the sample response, and the detector (microphone) response. The sample response can be obtained by a deconvolution process, which is a computationally intensive process and normally requires that the detector response be known. Once the sample response is determined, spectra for different time delays relative to the pulse can be determined.
Step scan phase modulated (PM) PAS experiments using digital signal processing (DSP) have been previously reported [Manning93], [Drapcho97]. These works used continuous PM with one or several discrete frequencies for sample excitation. That is, the retardation is stepped back and forth using a continuous square wave (say one laser wavelength in each direction about the nominal retardation at 100-400 Hz) for a sufficient duration to allow demodulation of the periodic components. From the resulting in-phase and quadrature data of the PM at each step, interferograms were created for different rotation angles between the two components to compute spectra at effectively different delays with respect to the excitation, giving the desired depth information.
Alternative approaches were shown by [Budevska96]. The first approach uses amplitude modulation (AM) with a shutter to generate a pulse of infrared light from the interferometer at each step and collect the PAS time response. This AM method has the same limitations in pulse mode as in continuous modulation in that it modulates the total of infrared light from the interferometer. Since the average value is much larger than the interferogram signal of interest, this reduces the resulting signal to noise ratio (S/N). The AM method has the additional limitation of having a very low effective pulse power level when the pulse length is set to achieve a useful resolution of a fraction of a millisecond, resulting in even lower S/N in the output result.
The second approach uses a slow rise time (12 ms) PM step for the sample excitation, with no indication of how to obtain the sample time response. This PM method proposed does not address the removal of the system function (system impulse response).